The Universal Mobile Telecommunication System (UMTS) is one of the 3G mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. The complete cellular system that comprises an LTE system (and thus E-UTRAN) is denoted Evolved Packet System (EPS). As illustrated in FIG. 1, a radio access network typically comprises user equipments (UE) 150a-b wirelessly connected to base stations (BS) 110a-c, commonly referred to as NodeB in UTRAN and eNodeB in E-UTRAN. Each BS 110a-c serves one or more areas each referred to as cells 120a-c. 
One of the hot topics for future developments of cellular systems is heterogeneous networks, also known as HetNets. Heterogeneous networks are networks with a mixed deployment of cells of widely different sizes, such as macro cells, micro cells, pico cells, and femto cells. The cells may either cover overlapping areas, e.g., when the area covered by a pico cell is also covered by a macro cell, or they may complement each other's coverage. Smaller cells, such as pico cells, will typically be overlapping with other cells. They will be used, e.g., to provide increased capacity at locations with dense user populations and high traffic volumes, so called hotspots, or to provide improved coverage, e.g., in terms of better channel quality at certain indoor locations.
An interesting scenario, illustrated in FIG. 2, is where an operator has licenses for both a Frequency Division Duplex (FDD) and a Time Division Duplex (TDD) spectrum in different spectrum bands. The operator may then deploy a macro cell 220a coverage, i.e., a macro cell layer, in the FDD spectrum, and smaller cells such as pico cells 220b-c, providing additional complementing or overlapping scattered coverage spots in the TDD spectrum. The pico cells thus form a TDD pico cell layer and the macro cells form an overlay FDD macro cell layer. In the scenario illustrated in FIG. 2, the TDD pico cells 220b-c are isolated from each other. This may however not always be the case.
The paired FDD spectrum is typically uplink (UL)/downlink (DL) symmetric, i.e., there is equal bandwidth assigned to UL and to DL communication. The TDD spectrum on the other hand is time-wise divided between UL and DL traffic, typically in an asymmetric fashion, where the DL is usually, although not always, assigned more capacity than the UL. The distribution of configured amounts of UL and DL resources is thus different in the paired FDD spectrum and in the TDD spectrum.
One problem, in particular with FDD, is that it is difficult to achieve a good matching of the distribution of configured UL/DL spectrum resources to an UL/DL distribution of the actual traffic. Heavy DL traffic may for instance more or less fully load the configured DL resources of a paired FDD spectrum, while there are still plenty of unused UL resources in the spectrum. The situation is slightly better for TDD due to its greater flexibility in reallocation of resources between UL and DL. However, even though TDD cells may be configured with different degrees of UL/DL resource asymmetry, the UL/DL distribution of the traffic may vary in a manner and on a time scale which is unrealistic to match with repeated reconfigurations of the distribution of UL/DL resources. Furthermore, reconfigurations of the UL/DL resources in adjacent TDD cells have to be synchronized to avoid severe UE to UE inter-cell interference, unless the TDD cells are completely isolated from each other. Such synchronization of the resource reconfigurations complicates dynamic reallocation of UL/DL resources in TDD scenarios.